The California Streets and Highways Code defines Route 10 from "(a) Route 1 in Santa Monica to Route 5 near Seventh Street in Los Angeles. (b) Route 101 near Mission Road in Los Angeles to the Arizona state line at the Colorado River via the vicinity of Monterey Park, Pomona, Colton, Indio, and Chiriaco Summit and via Blythe.." Despite the legislative definition, Caltrans connects the two sections of the route by cosigning I-10 down Interstate 5 between the East LA Interchange and the Santa Monica Freeway, negating a section of the San Bernardino Freeway west of I-5. This short section of Route 10 between Route 5 and Route 101, which was formerly defined as Route 110 (signed as Interstate 110) until 1968, is signed overhead for I-10 eastbound and for U.S. 101 westbound. This I-5/I-10 cosigning is consistent with the Federal Highway Administration's Interstate Highway route logs that such an overlap exists for the segment of I-10 in California.[5]

A typical traffic jam on the Santa Monica Freeway, at 2:30 p.m. on a Wednesday afternoon

At the East Los Angeles Interchange, State Route 60 diverges east towards Riverside and Pomona. I-10 then turns north, running concurrently with Interstate 5 for approximately one mile. Then, Interstate 10 heads east and merges with the traffic from the spur to US 101 onto the San Bernardino Freeway.[12][13]

The freeway is 14 lanes wide (two local, five express in each direction) from the Harbor Freeway (Interstate 110) interchange to the Arlington Avenue off-ramp. Most of these lanes are full at peak travel times (even on Saturdays). The remainder of the freeway varies between eight and 10 lanes in width. The whole freeway (though a much smaller version) opened in 1965, with a formal dedication held in 1966.[14]

While the construction of the Century Freeway several miles to the south reduced traffic congestion to a considerable amount by creating an alternate route from downtown to the Los Angeles International Airport, the Santa Monica Freeway is still one of the busiest freeways in the world. All three freeway-to-freeway interchanges along its length are notorious for their congestion, and are routinely ranked among the top 10 most congested spots in the United States.[citation needed]

Due to the high traffic volume, car accidents are so common that Caltrans has constructed special Accident Investigation Sites separated from the freeway by fences. These enable the California Highway Patrol to quickly clear accidents from the through traffic lanes, and the fences reduce congestion by preventing rubbernecking (in which vehicles slow down so their occupants can watch the accident investigation).[15]

I-10 westbound is usually signed as towards San Bernardino and/or Los Angeles in the Mojave Desert. Eastbound, in the San Gorgonio Pass, the signage indicates "Indio, Other Desert Cities", and indicates "Blythe" after Indio — the first sign for Phoenix does not occur until Indio.[citation needed]

The route from Indio via Mecca to the Arizona state line near Blythe was defined in 1919 as pre-1964 Legislative Route 64. (Later extensions took LR 64 west along present State Route 74; a 1931 cutoff bypassed Mecca to the north.) LR 26 was extended west from San Bernardino to Los Angeles in 1931, running along an alignment south of the existing US 66/US 99.[20] Neither of these was a signed route until around 1932, when U.S. Route 60 was extended west from Arizona to Los Angeles, running along LR 64 to Indio, LR 26 (with US 99) to Beaumont, pre-1964 Legislative Route 19 to Pomona, and LR 26 to Los Angeles. (The original alignment of LR 26 ran roughly where State Route 60 now is west of Pomona, but an alignment close to present I-10 opened around 1934).[citation needed]

Thus, in 1931, what is now I-10 east of Los Angeles had been defined as LR 26 from Los Angeles to Indio and LR 64 from Indio to Arizona. It was signed as US 99 from San Bernardino to Indio, and US 60 came along around 1932 from Los Angeles to Pomona and from Beaumont to Arizona. U.S. Route 70 was extended west from Arizona ca. 1936 along the whole route to Los Angeles,[citation needed] and, between 1933 and 1942,[citation needed] US 99 moved from US 66 to present I-10 between San Bernardino and Los Angeles, forming a three-way concurrency between Pomona and Los Angeles. Old alignments and names include Valley Boulevard, Ramona Boulevard and Garvey Avenue.

I-10 holds the distinction of being the first freeway in Los Angeles. A four-mile section of today's freeway was built between 1933 and 1935 at a cost of $877,000. The "Ramona Boulevard" highway linked downtown Los Angeles to the communities of the southern San Gabriel Valley. The roadway, which opened on April 20, 1935, was dubbed the "Air Line route," and was seen as a major achievement in traffic design.[21]

The route east from Los Angeles was added to the Interstate Highway System on August 7, 1957. It was assigned the I-10 number on August 14, 1957, and the short piece west of I-5 was approved as I-110 on November 10, 1958.[1] By then, most if not all of the San Bernardino Freeway had been completed, and I-10 was signed along the existing freeway along with US 70, US 99, and part of US 60. Those three routes were all removed in the 1964 renumbering, leaving only I-10.

This road is signed only for the roads it feeds into: US 101 northbound and I-10 eastbound. It has only two interchanges between its ends: a westbound exit off of the spur at Mission Road immediately before merging with U.S. Route 101 northbound, and the eastbound exit for State Street and Soto Street before it merges onto I-10 eastbound—this one is numbered (as exit 19).[3] There is no access from the I-10 spur to I-5.[13]